In FIG. 7 there is shown a conventional type of air bag device (a partial sectional view) which comprises a breakable air bag cover 31, a retainer 32 for mounting the air bag device on a part of the vehicle, an inflator 33 for generating gas and an air bag 34 folded in a closed space A defined by the cover 31, the retainer 32 and the inflator 33.
In a vehicle collision, the inflator 33, in response to a signal from a collision detector (not illustrated), inflates the air bag 34 by jetting a large quantity of gas rapidly from a number of gas discharge holes 33a. The air bag 34, as it starts its rapid inflation, breaks the cover 31, and then continues to inflate into a thick disc form and absorbs an impact when the occupant is thrown forward against it.
Such a conventional air bag device is assembled and disposed on the vehicle body in accordance with the following procedures. First of all, the air bag 34, a ring 35 and the inflator 33 are all fastened to the retainer 32 by means of bolts 36. Then, the air bag 34 is folded to a size as small as possible in such a form as to be inflatable rapidly, and the cover 31 for the air bag 34 is fitted to the retainer 32. Fasteners 40, such as blind rivets, are inserted in openings from the outer side of the cover 31, with washers 39, and extend into the interior of the retainer 32. The rivets, upon deformation, clamp the cover firmly between the washers 31 and the retainer 32. The air bag device is fastened to a mounting member 38 by means of bolts 37. Numeral 41 designates a protecting cover or trim piece mounted on the vehicle body.
According to Examined Japanese Utility Model Publication No. 58-49887, it is also known to affix an air bag cover to a part of the vehicle body using bolts and nuts.
With the structure of FIG. 7, when the air bag 34 is inflated and breaks the cover, a very high force is applied to the air bag cover 31 in a direction that tends to dislodge it from the retainer 32. This high force acts against the rivets 40 (or bolts) which extend through the holes 42 in the cover 31 and produces remarkably high shearing loads on the cover in the proximities of the holes 42. Accordingly, it is necessary to use a large number of rivets (or bolts) in order to prevent the cover from being detached from the retainer when the bag inflates. The large number of rivets makes assembly of the cover to the retainer cumbersome, time-consuming and costly.